How do you insulate a fireplace?

If you mean by this question, "How do I stop the cold draft in the Winter I feel coming from the fireplace?", then I have an answer.

Reach up into the chimney throat from inside the firebox and find a wrought iron lever to close the fireplace flu. This alone will stop a good deal of the cold air seeping into the room by way of the chimney. Remember to open this flu again if you wish to build and light a fire in the fireplace. If you do not plan on any fireplace use you may find a greater ability to stop the cold draft by fixing a non flammable piece of insulation (fiberglass) in the throat of the flu before closing it..

While a fireplace and the crackling fire within it can be attractive and comforting when there is a chill in the air, there is no good purpose served in these modern times by the 16th Century Fireplace. Consider closing it up altogether.